OSLO (Reuters) - Greenhouse gases are at higher levels in
the atmosphere than at any time in at least 800,000 years,
according to a study of Antarctic ice on Wednesday that extends
evidence that mankind is disrupting the climate.

Carbon dioxide and methane trapped in tiny bubbles of air
in ancient ice down to 3,200 meters (10,500 ft) below the
surface of Antarctica add 150,000 years of data to climate
records stretching back 650,000 years from shallower ice
drilling.

"We can firmly say that today's concentrations of carbon
dioxide and methane are 28 and 124 percent higher respectively
than at any time during the last 800,000 years," said Thomas
Stocker, an author of the report at the University of Berne.

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Before the Industrial Revolution, levels of greenhouse
gases were guided mainly by long-term shifts in the earth's
orbit around the sun that have plunged the planet into ice ages
and back again eight times in the past 800,000 years.

The U.N. Climate Panel last year blamed human activities,
led by burning of fossil fuels that release heat-trapping
gases, for modern global warming that may disrupt water and
food supplies with ever more droughts, floods and heatwaves.

"The driving forces now are very much different from the
driving forces in the past when there was only natural
variation," Stocker told Reuters of the study in the journal
Nature by scientists in Switzerland, France and Germany.

The experts, working on the European Project for Ice Coring
in Antarctica, drilled down almost to bedrock in Antarctica.
They recovered layers of ice formed by compressed snow, which
can be counted much like the rings on trees.

DEEPER ICE

Stocker said Chinese and Australian scientists were
examining possibilities for drilling in parts of Antarctica
with even deeper ice, in some places 4,500 meters thick, that
could yield atmospheric records dating back 1.5 million years.

The study also found big natural shifts in carbon dioxide
levels. "We find very conspicuous natural oscillations of
carbon dioxide 770,000 years ago that bear the fingerprint of
abrupt climate change during ice ages," Stocker said.

And the Nature report also set a new record low for carbon
dioxide at 172 parts per million (ppm) in the atmosphere about
667,000 years ago, about 10 ppm below the previous known low
and giving an ancient natural range of 172 to about 300 ppm.

The study suggested that the low might be a sign that the
oceans once soaked up more carbon dioxide. Carbon dioxide
levels are now at about 380 ppm.

Taken together, the data "allow us to learn more eventually
about the carbon cycle and its responses to climate change."

Temperatures in an ice age are about 5-6 Celsius (9 to 11
Fahrenheit) colder than now, already a mild period in earth's
history. Climate change could add a "best guess" of 1.8 to 4.0
Celsius this century, according to the U.N. panel.

The study also linked variations in methane to monsoons.

"The variations of methane concentration point to a
strengthening of the monsoon system in the tropics in the most
recent 400,000 years. These monsoon cycles have become stronger
in the second half of this long time period," Stocker said.